Bone Morphogenetic Protein 4 in the Extraembryonic Mesoderm Is
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Bone morphogenetic protein 4 in the extraembryonic mesoderm is required for allantois development and the localization and survival of primordial germ cells in the mouse Takeshi Fujiwara*, N. Ray Dunn*†, and Brigid L. M. Hogan*‡§ *Department of Cell Biology, Vanderbilt University Medical Center, and ‡Howard Hughes Medical Institute, 1161 21st Avenue South, Nashville, TN 37232-2175 Communicated by R. L. Brinster, University of Pennsylvania, Philadelphia, PA, September 26, 2001 (received for review July 3, 2001) Evidence suggests that the specification of primordial germ cells a cumulative process, initiated by signaling between the tropho- (PGCs) in the mammalian embryo does not depend on maternal blast and the inner cell mass (ICM) of the blastocyst and determinants. Rather, previous genetic analysis in the mouse has continued between the ExE and the proximal epiblast. shown that bone morphogenetic protein 4 (Bmp4) is required for The idea that interactions between the ExE and epiblast are the formation of both PGCs and allantois. Bmp4 is expressed in required for PGC development is supported by experimental and both the trophoblast-derived extraembryonic ectoderm (ExE) and genetic evidence. For example, if epiblast cells of the prestreak in the epiblast-derived extraembryonic mesoderm (ExM), in which embryo are cultured in contact with the ExE, some of the the PGCs, allantois primordium, and angioblasts are first detected. epiblast cells differentiate into AP-positive, presumptive PGCs We have shown that Bmp4 made in the ExE functions to induce (4). Moreover, studies suggest that this interaction is mediated precursors of PGCs and allantois in the adjacent epiblast, resulting by extracellular signaling proteins, including bone morphoge- in complete lack of both cell types in homozygous null mutants. netic proteins (Bmps) Bmp4 and Bmp8b, both of which are However, the function of Bmp4 in the ExM is totally unknown. To expressed in the ExE. Thus, Bmp4 homozygous null mutants address this question, we generated tetraploid (4N) chimeras by completely lack PGCs and an allantois, whereas Bmp8b homozy- aggregating Bmp4 null ES cells with wild-type tetraploid embryos. gous null mutants have either a significantly reduced number of In this combination, wild-type tetraploid cells contribute to the PGCs or none at all, and poor development of the allantois (5–7). extraembryonic trophoblast and primitive endoderm lineages but Culture of distal epiblasts with cells expressing Bmp4 and Bmp8b are excluded from the epiblast and its derivatives, including the results in the appearance of AP-positive, presumptive PGCs ExM. Our results clearly demonstrate that Bmp4 made in the ExM after about 72 h (8). does not affect the establishment of either PGC or allantois In addition to its expression in the ExE, Bmp4 is also tran- lineages, but is required for PGC localization and survival and for scribed in the extraembryonic mesoderm (ExM). At the late the differentiation of the allantois. These findings suggest that streak stage, Bmp4-expressing cells are predominantly localized Bmp4 expressed in epiblast-derived tissues plays vital roles in at the periphery of the incipient allantois bud, abutting the reproduction by regulating both the development of the germ line exocoelom and separated from the cluster of PGCs by a popu- and the vascular connection between the embryo and the placenta. lation of non-Bmp4-expressing mesenchyme cells. By the head- fold stage, Bmp4-expressing cells are more extensively distrib- n the mouse embryo, primordial germ cells (PGCs) are first uted within the allantois, and some are seen adjacent to the BIOLOGY PGCs (ref. 5 and Fig. 4 B and C). Taken together, these results Idetected at the head-fold stage as a cluster of about 45 alkaline DEVELOPMENTAL phosphatase (AP)-expressing cells at the base of the allantois suggest that Bmp4 has the potential to affect PGC and allantois (1). These PGCs subsequently migrate into the endoderm that development at several different steps: first, in a dose-dependent gives rise to the hindgut (2), distribute along the hindgut, and way in the establishment of precursors in the epiblast, then in the finally move through the dorsal mesentery into the genital ridges allocation of these precursors to the PGC or allantois lineages, (reviewed in ref. 3). Lineage analysis has shown that the PGCs and finally in the survival and differentiation of these cell types are derived before gastrulation from a subpopulation of proxi- once they have been specified. mal epiblast cells adjacent to the trophoblast-derived extraem- In this paper, we test the last two hypotheses by performing lacZ/tm1 bryonic ectoderm (ExE). Significantly, these precursors are not tetraploid chimera analysis. Lines of Bmp4 null embryonic lineage restricted but have descendants in both the germ line and stem (ES) cells were established and aggregated with wild-type extraembryonic tissues, such as the allantois, yolk sac mesoderm, tetraploid (4N) morulae to generate chimeric embryos (9). With and amnion (1). this strategy, the trophoblast and ExE will express Bmp4 (and Previous studies suggest that the specification of the mouse Bmp8b), and the primitive endoderm-derived visceral endoderm germ line does not involve, as in many other organisms, the inheritance of maternal determinants. Rather, it appears that Abbreviations: Bmp, bone morphogenetic protein; PGC, primordial germ cell; ExE, ex- interactions between the trophoblast-derived ExE and the ad- traembryonic ectoderm; ExM, extraembryonic mesoderm; ES, embryonic stem; 4N, tet- jacent epiblast first establish precursors that can give rise to both raploid; AP, alkaline phosphatase; ICM, inner cell mass; VCAM-1, vascular cell adhesion PGCs and extraembryonic mesoderm cells. A second process molecule 1. then allocates descendants of these cells to the different lineages †Present address: Department of Molecular and Cellular Biology, Harvard University, (reviewed in ref. 3). This model says nothing about whether the Cambridge, MA 02138. interactions induce the de novo expression of genes specifically §To whom reprint requests should be addressed at: Department of Cell Biology, Vanderbilt University Medical Center, C-2310 MCN, Nashville, TN 37232-2175. E-mail: brigid.hogan@ required for PGC development, or whether they maintain in the mcmail.vanderbilt.edu. PGCs expression of key factors permitting pluripotency that are The publication costs of this article were defrayed in part by page charge payment. This normally lost in cells that differentiate into somatic lineages. article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Moreover, the induction of the PGC͞allantois precursors may be §1734 solely to indicate this fact. www.pnas.org͞cgi͞doi͞10.1073͞pnas.241508898 PNAS ͉ November 20, 2001 ͉ vol. 98 ͉ no. 24 ͉ 13739–13744 Downloaded by guest on September 30, 2021 will express Bmp2 (7), but the ICM derivatives, including the ExM, will completely lack Bmp4 expression. We find that these chimeric embryos do contain PGCs at the presomite stage, in about the same number as control chimeras. However, most of the PGCs are incorrectly localized in the yolk sac, from which they later disappear. Mutant tetraploid chimeras also show abnormal development of the allantois, which fails to extend and fuse with the chorion and lacks a normal vasculature. We conclude that Bmp4 expressed in the ExM is not required for the allocation of precursors into the PGC and allantois͞ExM lin- eages but does affect the subsequent behavior and survival of the PGCs and the differentiation of the allantois. Materials and Methods Excision of the Floxed neor Cassette in Bmp4lacZneo ES Cells and Generation of Transheterozygous Bmp4 Null ES Cells. 12C Bmp4lacZneo ES cells (50 ϫ 106) (5) were electroporated with 100 g of cytomegalovirus (pCMV)-Cre plasmid. The neo excised clones were screened by Southern blot analysis of SpeI-digested genomic DNA with the 5Ј external 500-bp BsmI–BamHI Bmp4 genomic probe (10). One clone, 10G, was chosen for the generation of transheterozygous Bmp4 null ES cell lines. 10G ES cells (16.4 ϫ 106) were electroporated with 50 gofNotI- linearized Vb replacement vector, which targets exons 1 and 2 of the Bmp4 allele with a neo (neomycin-resistance) gene (10). Clones were screened by Southern blot analysis of XbaI-digested genomic DNA with the 5Ј external Bmp4 genomic probe as described. Five of 142 clones were identified as Bmp4lacZ/tm1 transheterozygous null lines (data not shown). Two lines, 5H and 6H, were chosen for further investigation by chimeric analysis. The heterozygous Bmp4lacZ/ϩ line, 3E, was used as a control. Generation of Tetraploid Chimeric Embryos. Tetraploid embryos were produced by aggregating four-cell stage tetraploid embryos with 12–16 ES cells as described (9). -Galactosidase Staining and Detection of PGCs. -Galactosidase staining, histological analysis, and whole-mount AP staining of tetraploid chimeric embryos were performed as described (5). Fig. 1. Morphology and posterior differentiation of tetraploid chimeras. Whole-Mount Immunostaining. Whole-mount immunostaining was (Left) Bmp4lacZ/ϩ chimeras. (Right) Bmp4lacZ/tm1 chimeras. (A and B) Lateral performed as described (11). Embryos were exposed to primary views of five-somite (5S; A) and four-somite (4S; B) tetraploid chimeras after staining with 5-bromo-4-chloro-3-indolyl -D-galactoside (X-Gal) to reveal antibodies against Pecam1 (PharMingen), Flk1 (PharMingen), lacZ or vascular cell adhesion molecule 1 (VCAM-1) (PharMingen) Bmp4 expression. Both chimeras show expression in the allantois, amnion, lateral plate mesoderm, and heart. Note the well-formed, extended allantois at a dilution of 1:150. Horseradish peroxidase-conjugated don- in the control (A) and unextended allantois in the null mutant chimera (B). (C key anti-rat IgG (Jackson ImmunoResearch) was used as a and D) Section through posterior of seven- or eight-somite chimeras in a plane secondary antibody at a dilution of 1:1,000. Antibody binding similar to that indicated in A and B. Embryos were stained with X-Gal, and was detected with the use of diaminobenzidine hydrochloride sections were counterstained with eosin.